The invention relates generally to methods of forming photoconductive layers comprising primarily lead monoxide (PbO) and, more particularly to such layers suited for use as photoconductive targets in camera tubes, such as vidicons.
In general, photoconductive layers of this type include regions of different conductivity type material lying one behind the other in the direction of thickness of the layer. A p-n barrier or junction is formed at the boundary region between the adjacent zones of different conductivity.
Certain prior art techniques are known for the manufacture of photoconductive lead oxide layers. These include, for example, the techniques described in the following U.S. Patents issued to E. F. deHaan et al.:
______________________________________ 3,289,024 issued on November 29, 1966, 3,307,983 " March 7, 1967, 3,372,056 " March 5, 1968, and 3,444,412 " May 13, 1969. ______________________________________
In the manufacture of devices which incorporate such photoconductive lead monoxide layers, certain deficiencies in the prior art processing methods have resulted in inconsistent and particularly poor yield of devices having desired lag characteristics particularly at low light levels.
Camera tubes of the type described display lag effects which result from inherent delays in the response of the photoconductive target material to changing light levels. This photoconductive lag is governed by the quality of the target material, which in turn is largely determined by the methods of preparation.
The lag effects control the build up or decay of signal output after the application or removal of light from the input surface of the camera tube. On a television picture this is seen as a short term image retention. The lag is an important property in camera tube characteristics particularly at low light levels since lag increases with decreasing light level. Also, because of the low blue light sensitivity of such camera tubes, the selection of low lag lead monoxide tubes suitable for the blue channel in a three tube TV camera is very critical. Presently, lead monoxide type camera tubes for special applications such as the blue channel of a TV camera, are carefully selected and screened for adequate lag performance from lots of tubes which are manufactured. A very low yield of such tubes is found when presently known techniques are employed in their manufacture.
A reliable method is desired for manufacturing devices having photoconductive lead monoxide layers with significantly improved yield of devices having improved lag characteristics.